Integrated circuits typically have a power bus which supplies power to internal circuitry. Often the internal circuitry is capable of performing a plurality of functions, with each of the functions having different power requirements. In such situations, the integrated circuit may be provided with a switch to selectively couple an appropriate one of a plurality of power supplies to the power bus.
An EPROM (electrically-programmable, read-only-memory) is one example of an integrated circuit having a plurality of power supplies which are selectively coupled to a power bus. An EPROM includes an array of floating-gate memory cells arranged in rows and columns. The floating gate of a programmed memory cell is charged with electrons, and the electrons in turn render the source-drain path under the charged floating gate nonconductive when a chosen row-line select voltage is applied to the control gate. The nonconductive state is read as a "zero" bit. The floating gate of a non-programmed cell is neutrally charged such that the source-drain path under the non-programmed floating gate is conductive when the same chosen row-line select voltage is applied to the control gate. The conductive state is read as a "one" bit.
Each column and row of an EPROM array may contain thousands of cells. The sources of cells in a column are connected to a virtual-ground line (source-column line). The drains of cells in a column are connected to a bitline (drain-column line). The control gates of each cell in a row are connected to a wordline.
During cell programming, appropriate programming voltages are applied to the selected control-gate wordline, the selected source-column line, and the selected drain-column line to create a high-current condition in the selected channel region, injecting channel-hot electrons and/or avalanche-breakdown electrons across the channel oxide to the floating gate.
During cell reading, appropriate reading voltages are applied to the selected control-gate wordline, the selected source-column line, and the selected drain-column line and the conductive or nonconductive state of the cell detected.
Appropriate programming and reading voltages are supplied to a selected wordline by a power bus. The power bus is selectively coupled by a power supply switch to a first high voltage power source during programming and to a second lower voltage power source during reading. Previous power supply switches have used field-effect transistors formed in the face of an integrated circuit.